1. Field of the Invention
The present invention relates to a radiation therapy treatment planning (hereafter referred to as RTP) machine to plan for cancer treatment with radiation therapy. The present invention more specifically relates to an RTP machine preferably used for treatment planning in cases where radiation is applied while changing the radiation field by a multileaf collimator.
2. Description of the Related Art
Radiation therapy is one method of cancer treatment utilizing radiation. In radiation therapy it is preferable that high radiation dosage is irradiated on the target, and radiation dosage as low as possible is irradiated on organs other than the target. Therefore, a treatment plan is necessary for applying radiation therapy. A machine to deliver this treatment plan is referred to as an RTP machine.
FIG. 6 is a perspective view showing the constitution of a radiation therapy treatment machine called a linear accelerator (hereafter referred to as linac). The linac includes a gantry 61, a collimator 62, a rotation shaft 64 for the gantry 61, a couch 65, and a rotation shaft 66 for the couch 65. Gantry 61 incorporates a beam source in its head, and rotates about rotation shaft 64 in the directions indicated by arrow A in the drawing. Collimator 62 is connected with the head of gantry 61, and rotates in the directions indicated by arrow B in the drawing. Collimator 62 incorporates a multileaf collimator, and the radiation ray from the radiation source inside gantry 61 is masked to any field shape, and then, irradiates toward couch 65. Couch 65 is utilized to support the patient, and rotates about couch rotation shaft 66 in the directions of C in the drawing. Couch rotation shaft 66 coincides with a vertical line in a state where collimator 62 is facing downward. In general, the gantry angle 0 degrees is defined in the linac when collimator 62 is facing directly downward, and the angle increases clockwise. FIG. 6 shows where the gantry angle is 0 degrees. In general, a linac can rotate from 180 degrees to 180 degrees passing through 0 degrees. When the radiation ray irradiates the target, it is necessary to irradiate from a direction that avoids critical organs such as the eyeball and the spinal cord by changing the gantry angle, the collimator angle, and the couch angle.
FIG. 7 is a perspective view showing the construction of a multileaf collimator (hereafter referred to as MLC). In this drawing, the MLC includes upper jaws 71, lower jaws 72, and leaves 73. Upper jaws 71 move to open and close in directions indicated by arrows L1 and L2. Lower jaws 72 include a number of leaves 73, and move to open and close in directions W1 and W2. The individual leaves 73 move independently to alter the radiation field shape. The intersection between gantry rotation shaft 64 and a perpendicular line extending down from the beam source is referred to as an isocenter. A plane which has this isocenter as a center is in line with the gantry rotation shaft 64, and crosses at right angles with a perpendicular line extending down from the gantry head is referred to as an isocenter plane. An isocenter is often used as a reference point for dosage calculation, and a beam usually irradiates such that the center of the target would be at the isocenter. The field shape of radiation usually refers to this shape on the isocenter plane.
FIG. 8 shows a sectional view representing the radiation field on the isocenter plane formed by the MLC. The drawing indicates the upper jaws 71, the lower jaws 72, the location of the pointed radiation beam source 85 of the linac, the isocenter plane 86, the center axis of the radiation beam 84, and the radiation field 87. The beam emitted from beam source 85 forms radiation field 87 on isocenter plane 86, blocking the ray by using upper jaws 71 and leaves 73 of lower jaws 72.
The radiation therapy technique, where the isocenter is positioned inside the patient's body and irradiates with a fixed gantry, is referred to as the SAD technique. The radiation therapy technique, where the isocenter is positioned on the patient skin, is referred to as the SAD technique. The radiation therapy technique, which irradiates the target while the gantry is rotating, is referred to as rotational therapy. The isocenter is usually positioned inside the patient's body with the rotational therapy. The radiation therapy technique, where the radiation field matches the target shape with the fixed gantry, is referred to as irregular field radiation therapy. In addition, the radiation therapy technique, which irradiates with fixed beams while the gantry angle switches, is referred to as irregular field multi-beam radiation therapy. The radiation therapy technique, where the radiation field formed by the MLC matches the target shape by adjusting the leaf positions while the gantry is rotating to concentrate radiation on the target, is referred to as conformal therapy.
FIG. 9 shows how the radiation field shape of conformal therapy changes during the gantry rotation. Reference number 91 indicates the radiation field shape when the gantry angle is 216 degrees, reference number 92 indicates the radiation field shape when the gantry angle is 288 degrees, reference number 93 indicates the radiation field shape when the gantry angle is 0 degrees, reference number 94 indicates the radiation field shape when the gantry angle is 72 degrees, and reference number 95 indicates the radiation field shape when the gantry angle is 144 degrees. Conformal therapy is a method to rotate the linac beam source around the target, and simultaneously, to adjust the positions of the MLC installed at the beam source of the radiation ray so as to form the radiation field to correspond with the shape of the affected part. Actual examples to generate MLC positions from a target shape are disclosed in the Japanese Patent Application Publication H01-214343, Japanese Patent Application Publication H08-131566, and others.
The following section briefly describes how to generate the MLC positions. Computer Tomography, CT, images are examined to diagnose the location and the shape of the cancer. The CT images are used to confirm the location of the affected part in three dimensions by examining images usually at intervals of 1 cm around the affected part of the patient. Then, the contours of the affected part namely the cancer are extracted from this group of the CT images. For conformal therapy, the MLC leaf positions are usually calculated from contours which are extracted from the CT images projected in two dimensions seen from the gantry angle with a certain width of margin processing. In other words, while the gantry is rotating and irradiating, the shape of the cancer at the gantry angle is changing so that the leaves are being moved to fit the changes so as to alter the radiation field to dose radiation to the cancer effectively and to avoid irradiating to normal tissue as much as possible. This calculation for the MLC leaf positions is conducted by a Multileaf-Collimator-Position-Calculation-Unit.
IMRT is one type of irregular field multi-beam radiation therapy which has been utilized recently. IMRT is an abbreviation of Intensity Modulated Radiation Therapy, and is one radiation therapy technique used to modulate the beam intensity to irradiate unevenly in order to distribute optimal dosage. Actual examples of this beam intensity modulated radiation therapy are the step-and-shoot method and the sliding-window method.
FIG. 10 is a drawing showing the radiation field shapes by the MLC of the step-and-shoot method. In this drawing, reference numbers 101a, 101b, 101c, and 101d are segments of the radiation field shapes when the gantry angle is 216 degrees, reference number 102 indicates the radiation field shape when the gantry angle is 288 degrees, reference number 103 indicates the radiation field shape when the gantry angle is 0 degrees, reference number 104 indicates the radiation field shape when the gantry angle is 72 degrees, and reference number 105 indicates the radiation field shape when the gantry angle is 144 degrees. The step-and-shoot method is conducted as one irregular field multi-beam radiation therapy. The radiation field shapes at each angle consist of multiple radiation fields where the MLC forms discretionary shapes referred to as segments. In this example, four segments are used for each radiation angle, and each segment accumulates radiation dosage by turning the beam on and off. In the example in FIG. 10, where the gantry angle is 216 degrees, irradiation is provided with four patterns as segment 101a, 101b, 101c, and 101d. At other gantry angles, irradiation is similarly provided with different shapes of radiation fields at multiple times, and each dosage is accumulated in order to distribute optimal dosage.
FIG. 11 is a drawing showing the MLC radiation field shape of the sliding window method. In this drawing, reference number 111 indicates the radiation field shape when the gantry angle is 216 degrees, reference number 112 indicates the radiation field shape when the gantry angle is 288 degrees, reference number 113 indicates the radiation field shape when the gantry angle is 0 degrees, reference number 114 indicates the radiation field shape when the gantry angle is 72 degrees, and reference number 115 indicates the radiation field shape when the gantry angle is 144 degrees.
The sliding window method is conducted as one type of irregular field multi-beam radiation therapy, and irradiates with beams, the intensity of which are modulated by the MLC which continuously moves while irradiating. Reference numbers 111, 112, 113, 114, and 115 show how the MLC leaf positions change. In contrast to the step-and-shoot method, the beam is not turned on or off during one irradiation. This irradiation is conducted from each irradiation direction (gantry angle). This method is to distribute the optimal dosage by accumulating each series of irradiation at each angle with moving and controlling the MLC.
An RTP machine is constituted of hardware equipment such as a computer main unit, a keyboard, a monitor, a scanner, a printer, and software to control them. It plans how to operate the linac based on methods such as the aforementioned different types of radiation therapy, and creates data to control the gantry angle, the collimator angle, the MLC positions, the couch angle, the radiation dosage, and others. The created data is input into a linac, which operates according to the data, and a radiation therapy is provided as planned by the RTP machine.
In conformal therapy and IMRT sliding window method, the leaf positions change during the irradiation. An MLC is constituted of a number of metal leaves, and the individual leaves are driven by motors to control their positions. Therefore, when the leaves move, the leaf motion speed is limited by the motor revolution speed limit, and the leaf motion acceleration is also limited by the leaf momentum and the motor torque. When MLC leaf motion speed or acceleration exceeds the tolerance and then a leaf positioning error has occurred due to going over the acceptable range, the linac usually detects an error and deactivates itself. Though there are some types of linacs which are capable of resuming after the MLC positioning has been completed, in general the operator has to reset the treatment parameters according to the deactivated status. In either case, since radiation output of a linac cannot rise quickly, there would be an error with the treatment plan. In case of a pneumatic drive or a hydraulic drive to control the MLC, there would still be a speed and torque limit as with the motor drive.